OP -- I do understand that sometimes there are total breakdowns in inlaw relationships but I'm talking more about the "I can't stand my MIL because she gives my children sweets/criticises my dusting/sniffs at my cooking"

It's exactly that sort of petty, stupid, rude, thoughtless, and unnecessary comment that makes for deep and permanent rifts.

Would you speak your mind so freely to your MIL about her dusting or her cooking or other things she has managed on her own for many years? In what parallel universe is any of that acceptable?

If you spoke to others like that, in their own homes, what response would you anticipate? If you fed someone's pet something they had asked you not to feed it, how do you think they would feel?